Display apparatus and image pickup apparatus

ABSTRACT

A first light-emitting layer of a first organic electroluminescent element is disposed in common to a second organic electroluminescent element, a second light-emitting layer of the second organic electroluminescent element is disposed in contact with the first light-emitting layer and in the cathode side, and the second light-emitting layer is a light-emitting layer having an electron trapping property.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus provided with anorganic electroluminescent element.

2. Description of the Related Art

The organic electroluminescent element has been developed actively inrecent years and has a configuration in which an anode, an organiccompound containing at least a light-emitting layer, and a cathode arestacked. Regarding a general method for manufacturing a multicolordisplay apparatus including organic electroluminescent elements of threecolors, red, green, and blue, each of light-emitting layers of red,green, and blue is vacuum-evaporated by using a metal mask forpatterning in accordance with the pixel shape of the color concerned.

The pixel size of the display apparatus has been reduced, and a highdegree of accuracy has been required with respect to the metal mask forpatterning in accordance with the pixel shape. As a result, productionand maintenance of a metal mask with a high degree of accuracy havebecome difficult.

Japanese Patent Laid-Open No. 2007-066862 discloses a configuration inwhich a blue light-emitting layer is disposed all over a pixel regionand a red light-emitting layer and a green light-emitting layer aredisposed while being stacked as layers on the blue light-emitting layer.It is stated that the blue light-emitting layer is formed all over thepixel region without using a high-accuracy mask, so as to reduce thenumber of usage of the metal mask for patterning and, in addition, thelife of the display apparatus can be improved by increasing a blue pixelarea having a low luminous efficacy.

SUMMARY OF THE INVENTION

In the above-described configuration, regarding the red and the greenorganic electroluminescent elements, it is necessary that the bluelight-emitting layer disposed all over the pixel region is not allowedto emit light, but only the red and the green light-emitting layersstacked are allowed to emit light. However, in some cases, electronspass through the red light-emitting layer and the green light-emittinglayer depending on the configurations of the red light-emitting layerand the green light-emitting layer, the electrons are leaked to the bluelight-emitting layer, the blue light-emitting layer is allowed to emitlight and, thereby, it becomes difficult to allow the red light-emittinglayer and the green light-emitting layer to emit light efficiently.

Furthermore, Japanese Patent Laid-Open No. 2007-066862 discloses thatelectron block layers may be disposed between the red light-emittinglayer and the blue light-emitting layer and between the greenlight-emitting layer and the blue light-emitting layer. However,regarding the configuration in which a charge block layer, e.g., theelectron block layer, is disposed, the drive voltage of the elementincreases.

Aspects of the present invention provide a display apparatus including alight-emitting layer disposed in common to organic electroluminescentelements to emit different colors, wherein the individual organicelectroluminescent elements are allowed to emit light efficientlywithout disposing a charge block layer between light-emitting layers.

A display apparatus according to aspects of the present inventionincludes a first organic electroluminescent element to emit a firstcolor and a second organic electroluminescent element to emit a secondcolor different from the above-described first color, theabove-described organic electroluminescent element being provided withan anode, a cathode, and a light-emitting layer disposed between theabove-described anode and the above-described cathode, wherein a firstlight-emitting layer of the above-described first organicelectroluminescent element is disposed in common to the above-describedsecond organic electroluminescent element, a second light-emitting layerof the above-described second organic electroluminescent element isdisposed in contact with the above-described first light-emitting layerand in the side nearer to the above-described cathode than is theabove-described first light-emitting layer, and the above-describedsecond light-emitting layer contains a host material and alight-emitting dopant material and is configured to satisfy Relationalexpressions (A) and (B) described below,

|LUMO _(D2) |>|LUMO _(H2)|  (A)

|LUMO _(D2) |−|LUMO _(H2) |>|HOMO _(H2) |−|HOMO _(D2)|  (B)

where LUMO_(H2) and HOMO_(H2) represent the LUMO level energy and theHOMO level energy, respectively, of the above-described host materialcontained in the above-described second light-emitting layer andLUMO_(D2) and HOMO_(D2) represent the LUMO level energy and the HOMOlevel energy, respectively, of the above-described light-emitting dopantmaterial contained in the above-described second light-emitting layer.

According to aspects of the present invention, regarding a displayapparatus including a light-emitting layer disposed in common to organicelectroluminescent elements to emit different colors, the individualorganic electroluminescent elements are allowed to emit lightefficiently without disposing a charge block layer betweenlight-emitting layers.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams showing an example of a displayapparatus according to a first embodiment.

FIGS. 2A and 2B are schematic diagrams showing energy bands oflight-emitting layers of a second organic electroluminescent elementaccording to the first embodiment.

FIG. 3 is a schematic diagram showing an example of a display apparatusaccording to a second embodiment.

FIGS. 4A and 4B are schematic diagrams showing energy bands oflight-emitting layers of a third organic electroluminescent elementaccording to the second embodiment.

DESCRIPTION OF THE EMBODIMENTS

A display apparatus according to aspects of the present invention willbe described below on the basis of embodiments with reference to thedrawings. Regarding the portions not specifically shown in the drawingsor described in the present specification, well-known or publicly knowntechnologies in the related art are applied. The embodiments explainedbelow are no more than examples, and the present invention is notlimited to them.

In particular, in the following embodiments, a first color, a secondcolor, and a third color are specified to be green, red, and blue,respectively. A first organic electroluminescent element, a secondorganic electroluminescent element, and a third organicelectroluminescent element are specified to be a green organicelectroluminescent element, a red organic electroluminescent element,and a blue organic electroluminescent element. A first light-emittinglayer, a second light-emitting layer, and a third light-emitting layerare specified to be a green light-emitting layer, a red light-emittinglayer, and a blue light-emitting layer, respectively. However, thepresent invention is not limited to this configuration.

FIG. 1A is a schematic perspective diagram showing a display apparatusaccording to a first embodiment. The display apparatus according to thepresent embodiment includes a plurality of pixels 1 provided with anorganic electroluminescent element. The plurality of pixels 1 arearranged in the matrix, so as to constitute a display region 2. In thisregard, the pixel refers to a region corresponding to a light-emittingregion of one light-emitting element. In the display apparatus accordingto the present embodiment, the light-emitting element is an organicelectroluminescent element and one color of organic electroluminescentelement is disposed in each pixel 1. Examples of colors emitted from theorganic electroluminescent element include red, green, blue, yellow,cyan, magenta, and white. In the display apparatus according to thepresent embodiment, a plurality of pixel units formed from a pluralityof pixels having different emission colors (for example, a pixel to emitred, a pixel to emit green, and a pixel to emit blue) are arranged. Thepixel unit refers to a minimum unit which can emit a predetermined colorthrough color mixing of the individual pixels.

FIG. 1B is a schematic partial sectional diagram along a line IB-IBshown in FIG. 1A. The pixel 1 is formed from an organicelectroluminescent element 3R (3G, 3B) provided with an anode 11, a holetransportation layer 12, a light-emitting layer 13R (13G, 13B)containing an organic compound, an electron transportation layer 14, anda cathode 15 on a substrate 10. The organic electroluminescent element3R is an organic electroluminescent element to emit red and the redlight-emitting layer 13R in the element emits light. Likewise, theorganic electroluminescent elements 3G and 3B are an organicelectroluminescent element to emit green and an organicelectroluminescent element to emit blue, respectively, and the greenlight-emitting layer 13G and the blue light-emitting layer 13B,respectively, in the elements emit light.

The anode 11 is disposed separately from the anodes 11 of adjacentpixels, and an insulating layer 20 is disposed between pixels (moreconcretely, anodes 11) in order to prevent an occurrence ofshort-circuit with the cathode 15 due to a foreign substance. The holetransportation layer 12, the electron transportation layer 14, and thecathode 15 may be disposed in common to the adjacent pixels, as shown inFIG. 1B, or be disposed on a pixel basis through patterning.

The individual organic electroluminescent elements are sealed with aseal cap 30 in such a way that external oxygen and moisture do notenter. A desiccant is included in the inside of the seal cap 30.

In the present embodiment, the green light-emitting layer 13G of thegreen organic electroluminescent element 3G is integrally disposed overthe regions of the organic electroluminescent elements 3R and 3B and,therefore, the green light-emitting layer 13G serves as a so-calledcommon light-emitting layer. According to this configuration, the numberof usage of a high-accuracy metal mask for patterning the light-emittinglayer can be reduced.

Furthermore, in the red organic electroluminescent element 3R, the redlight-emitting layer 13R is disposed while being stacked in contact withthe green light-emitting layer 13G and in the cathode 15 side of thegreen light-emitting layer 13G. Likewise, in the blue organicelectroluminescent element 3B, the blue light-emitting layer 13B isdisposed while being stacked in contact with the green light-emittinglayer 13G and in the cathode 15 side of the green light-emitting layer13G. That is, in the configuration of the present embodiment, a chargeblock layer is not disposed between the red light-emitting layer 13R andthe green light-emitting layer 13G serving as the common light-emittinglayer nor between the blue light-emitting layer 13B and the greenlight-emitting layer 13G serving as the common light-emitting layer.

In order to allow the red organic electroluminescent element 3R and theblue organic electroluminescent element 3B to emit light efficientlyeven in the configuration in which a charge block layer is not disposed,the configurations of the red light-emitting layer 13R and the bluelight-emitting layer 13B are devised. That is, in the presentembodiment, the red light-emitting layer 13R contains a host materialand a light-emitting dopant material and is configured to satisfyRelational expressions (1) and (2) described below,

|LUMO _(D2) |>|LUMO _(H2)|  (1)

|LUMO _(D2) |−|LUMO _(H2) |>|HOMO _(H2) |−|HOMO _(D2)|  (2)

where LUMO_(H2) and HOMO_(H2) represent the lowest unoccupied molecularorbital (LUMO) level energy and the highest occupied molecular orbital(HOMO) level energy, respectively, of the host material contained in thered light-emitting layer 13R, and LUMO_(D2) and HOMO_(D2) represent theLUMO level energy and the HOMO level energy, respectively, of thelight-emitting dopant material contained in the red light-emitting layer13R.

Likewise, the blue light-emitting layer 13B contains a host material anda light-emitting dopant material and is configured to satisfy Relationalexpressions (3) and (4) described below,

|LUMO _(D3) |>|LUMO _(H3)|  (3)

|LUMO _(D3) |−|LUMO _(H3) |>|HOMO _(H3) |−|HOMO _(D3)|  (4)

where LUMO_(H3) and HOMO_(H3) represent the LUMO level energy and theHOMO level energy, respectively, of the host material contained in theblue light-emitting layer 13B and LUMO_(D3) and HOMO_(D3) represent theLUMO level energy and the HOMO level energy, respectively, of thelight-emitting dopant material contained in the blue light-emittinglayer 13B.

FIGS. 2A and 2B show energy bands of the red light-emitting layer 13Rsatisfying Relational expressions (1) and (2) or the blue light-emittinglayer 13B satisfying Relational expressions (3) and (4). Hereafter andin FIGS. 2A and 2B, the term “LUMO_(D)” represents LUMO_(D2) orLUMO_(D3), although the subscript, “2” or “3”, is omitted. The same goesfor HOMO_(D) and the like.

In FIG. 2A, the absolute value of the LUMO_(D) of the light-emittingdopant material is larger than the absolute value of the LUMO_(H) of thehost material (|LUMO_(D)|>|LUMO_(H)|). Consequently, when electrons areinjected from the electron transportation layer 14 into thelight-emitting layers 13R and 13B, electrons are trapped at the LUMOlevel of the light-emitting dopant material and do not move in thelight-emitting layers 13R and 13B easily to reach the greenlight-emitting layer 13G serving as the common light-emitting layer.Meanwhile, the absolute value of the HOMO_(D) of the light-emittingdopant material is larger than the absolute value of the HOMO_(H) of thehost material (|HOMO_(D)|>|HOMO_(H)|). Consequently, when holes areinjected from the green light-emitting layer 13G into the light-emittinglayers 13R and 13B, holes are not trapped by the light-emitting dopantmaterial and move in the light-emitting layers 13R and 13B. As a result,the light-emitting layers 13R and 13B have a so-called electron trappingproperty to move holes without moving electrons, and recombine electronsand holes efficiently in the light-emitting layers 13R and 13B, so thatit becomes possible to use the recombination energy for emission oflight.

In FIG. 2B, LUMO_(D)|>|LUMO_(H)| holds as in FIG. 2A. However, theabsolute value of the HOMO_(D) of the light-emitting dopant material issmaller than the absolute value of the HOMO_(H) of the host material(|HOMO_(D)|<|HOMO_(H)|). Consequently, when holes are injected from thegreen light-emitting layer 13G into the light-emitting layers 13R and13B, holes are trapped at the HOMO level of the light-emitting dopantmaterial easily. However, the energy difference |LUMO_(D)|−|LUMO_(H)|representing the strength of the electron trapping property is largerthan the energy difference |HOMO_(H)|−|HOMO_(D)| representing thestrength of the hole trapping property(|LUMO_(D)|−|LUMO_(H)|>|HOMO_(H)|−|HOMO_(D)|). As a result, thelight-emitting layers 13R and 13B have the electron trapping property tosuppress the movement of electrons, so that it becomes possible torecombine electrons and holes in the light-emitting layers 13R and 13Befficiently.

In particular, the configuration shown in FIG. 2A can be employed. Inthis case, the red light-emitting layer 13R satisfies Relationalexpression (5) described below and the blue light-emitting layer 13Bsatisfies Relational expression (6) described below.

|HOMO _(D2) |>|HOMO _(H2) |>|LUMO _(D2) |>|LUMO _(H2)|  (5)

|HOMO _(D3) |>|HOMO _(H3) |>|LUMO _(D3) |>|LUMO _(H3)|  (6)

In the case where the light-emitting layers 13R and 13B disposed incontact with the green light-emitting layer 13G serving as the commonlight-emitting layer and in the cathode 15 side satisfy Relationalexpressions described above, leakage of electrons to the commonlight-emitting layer is prevented and, thereby, the organicelectroluminescent elements 3R and 3B are allowed to emit lightefficiently. In this regard, in the red organic electroluminescentelement 3R and the blue organic electroluminescent element 3B, thecommon light-emitting layer does not emit light in spite of being calleda common light-emitting layer. That is, in the red organicelectroluminescent element 3R, only the red light-emitting layer 13Remits light and in the blue organic electroluminescent element 3B, onlythe blue light-emitting layer 13B emits light. In aspects of the presentinvention, the term “do not emit light” refers to emit completely nolight or emit light having intensity only at a level at which theintensity is not visually identified.

In the present embodiment, the green light-emitting layer 13G ismentioned as an example of the common light-emitting layer, although notspecifically limited to this. As for the common light-emitting layer,another color light-emitting layer, e.g., a blue light-emitting layer13B or a red light-emitting layer 13R, may also be applied. For example,in the case where the blue light-emitting layer 13B serves as a commonlight-emitting layer, the red light-emitting layer 13R may be configuredto satisfy Relational expressions (1) and (2) described above, and thegreen light-emitting layer 13G may be configured to satisfy Relationalexpressions (3) and (4) described above.

In the configuration of the present embodiment, the anode 11, the holetransportation layer 12, the light-emitting layer, the electrontransportation layer 14, and the cathode 15 are stacked in that orderfrom the substrate 10 side. However, conversely, the cathode 15, theelectron transportation layer 14, the light-emitting layer, the holetransportation layer 12, and the anode 11 may be stacked in that orderfrom the substrate 10 side.

The display apparatus according to aspects of the present invention maybe a bottom emission type display apparatus in which the light of theorganic electroluminescent element is emitted from the substrate 10side, or be a top emission type display apparatus in which the light ofthe organic electroluminescent element is emitted from the side oppositeto the substrate 10.

Next, the individual members will be described concretely.

As for the substrate 10, for example, an insulating substrate made fromglass, plastic, or the like and a silicon substrate may be used. In thesubstrate 10, switching elements, e.g., transistors and MIM elements,may be disposed on the above-described insulating substrate or the like.In that case, the substrate 10 may have a flattening film to flattenunevenness due to the switching elements.

As for the anode 11 and the cathode 15, for example, transparent oxideelectrically conductive layers of tin oxide, indium oxide, indium tinoxide, indium zinc oxide, and the like and metal layers made from metalsimple substances, e.g., Al, Ag, Cr, Ti, Mo, W, Au, Mg, and Cs, oralloys thereof may be used. Furthermore, the anode 11 and the cathode 15may be formed from a stacked film of the transparent oxide electricallyconductive layer and the metal layer or a stacked film of a plurality ofmetal layers.

The hole transportation layer 12 is formed from a single layer or aplurality of layers of an organic compound provided with a holeinjection property and a hole transportation property. Meanwhile, theelectron transportation layer 14 is formed from a single layer or aplurality of layers of an organic compound provided with an electroninjection property and an electron transportation property. Optionally,in order to prevent movement of electrons from the light-emitting layerto the anode 11 side, an electron block layer may be disposed as thehole transportation layer 12. A hole block layer may be disposed as theelectron transportation layer 14. An exciton block layer to suppressdiffusion of excitons generated in the light-emitting layer may bedisposed as the hole transportation layer 12 or the electrontransportation layer 14. In this regard, the hole transportation layer12 and the electron transportation layer 14 are not indispensable andmay be omitted depending on the configuration of the organicelectroluminescent element.

The material for the light-emitting layer is not specifically limitedand a known material may be applied. The light-emitting dopant materialmay be either a fluorescent material or a phosphorescent material. Thegreen light-emitting layer 13G serving as a common light-emitting layermay be formed from only a light-emitting material or be a mixed layer ofa light-emitting dopant material and a host material. Furthermore, thelight-emitting layer may contain an assist dopant material besides thehost material and the light-emitting dopant material. In aspects of thepresent invention, the host material refers to a material having alargest content on a weight basis among the components in thelight-emitting layer.

As for the insulating layer 20, resin materials, e.g., acrylic resinsand polyimide resins, and inorganic materials, e.g., silicon nitride,may be used. Furthermore, a stacked film of the resin material and theinorganic material may also be used. The insulating layer 20 is notindispensable and may be omitted insofar as an occurrence ofshort-circuit between the anode 11 and the cathode 15 is prevented inthe configuration.

As for the seal cap 30, a cap-shaped member of glass, plastic, or thelike may be used. The seal cap 30 may be formed from, for example, atabular member, e.g., a glass plate, and a sealing agent disposed aroundthe display region 2 in order to bond the member and the substrate 10. Agas, e.g., nitrogen or argon, may be sealed into a space between theseal cap 30 and the cathode 15 of the organic electroluminescentelement, or the space may be filled with a resin material, e.g., anacrylic resin.

Any configuration to seal the organic electroluminescent element may beemployed. Regarding the configuration, in place of the seal cap 30, aseal film made from an inorganic material, e.g., silicon nitride,silicon oxide, or aluminum oxide, may be disposed on the cathode 15 ofthe organic electroluminescent element. The seal film may be formed froma stacked film of at least two layers of inorganic materials or beformed from a stacked film of an inorganic material and a resinmaterial.

The display apparatus according to aspects of the present invention isused in display portions of television systems and personal computers.In addition, the display apparatus may be used in display portions andelectronic viewfinders of image pickup apparatuses, e.g., digitalcameras and digital video cameras. The image pickup apparatus furtherincludes image pickup elements, e.g., an image pickup optical system anda CMOS sensor, to pick up an image.

The display apparatus according to the present embodiment may be used ina display portion of a cellular phone, a display portion of a hand-heldvideo game machine, and the like and, furthermore, be used in a displayportion of a portable music player, a display portion of a personaldigital assistant (PDA), and a display portion of a car navigationsystem.

FIG. 3 is a schematic partial sectional diagram showing a secondembodiment according to aspects of the present invention. Theconfiguration of the present embodiment is the same as the configurationof the first embodiment except that the blue light-emitting layer 13B isdisposed in contact with the green light-emitting layer 13G while beingstacked in the anode 11 side and the configuration of the bluelight-emitting layer 13B is different. In the red organicelectroluminescent element 3R, only the red light-emitting layer 13Remits light and in the blue organic electroluminescent element 3B, onlythe blue light-emitting layer 13B emits light.

The blue light-emitting layer 13B contains a host material and alight-emitting dopant material and is configured to satisfy Relationalexpressions (7) and (8) described below,

|HOMO _(D3) |<|HOMO _(H3)|  (7)

|HOMO _(H3) |−|HOMO _(D3) |>|LUMO _(D3) |−|LUMO _(H3)|  (8)

where LUMO_(H3) and HOMO_(H3) represent the LUMO level energy and theHOMO level energy, respectively, of the host material contained in theblue light-emitting layer 13B and LUMO_(D3) and HOMO_(D3) represent theLUMO level energy and the HOMO level energy, respectively, of thelight-emitting dopant material contained in the blue light-emittinglayer 13B.

FIGS. 4A and 4B show energy bands of the blue light-emitting layer 13Bsatisfying Relational expressions (7) and (8). Hereafter and in FIGS. 4Aand 4B, the term “LUMO_(D)” represents LUMO_(D3), although the subscript“3” is omitted. The same goes for HOMO_(D) and the like.

In FIG. 4A, the absolute value of the HOMO_(D) of the light-emittingdopant material is smaller than the absolute value of the HOMO_(H) ofthe host material (|HOMO_(D)|<|HOMO_(H)|). Consequently, when holes areinjected from the hole transportation layer 12 into the bluelight-emitting layer 13B, holes are trapped at the HOMO level of thelight-emitting dopant material and do not move in the bluelight-emitting layer 13B easily to reach the green light-emitting layer13G serving as the common light-emitting layer. Meanwhile, the absolutevalue of the LUMO_(D) of the light-emitting dopant material is smallerthan the absolute value of the LUMO_(H) of the host material(|LUMO_(D)|<|LUMO_(H)|). Consequently, when electrons are injected fromthe green light-emitting layer 13G into the blue light-emitting layer13B, electrons are not trapped by the light-emitting dopant and move inthe blue light-emitting layer 13B. As a result, the blue light-emittinglayer 13B has a so-called hole trapping property to move electronswithout moving holes, and recombine electrons and holes efficiently inthe blue light-emitting layer 13B, so that it becomes possible to usethe recombination energy for emission of light.

In FIG. 4B, |HOMO_(D)|<|HOMO_(H)| holds as in FIG. 4A. However, theabsolute value of LUMO_(D) of the light-emitting dopant material islarger than the absolute value of LUMO_(H) of the host material(|LUMO_(D)|>|LUMO_(H)|). Therefore, when electrons are injected from thegreen light-emitting layer 13G into the blue light-emitting layer 13B,electrons are trapped at the LUMO level of the light-emitting dopantmaterial easily. However, the energy difference |HOMO_(H)|−|HOMO_(D)|representing the strength of the hole trapping property is larger thanthe energy difference |LUMO_(D)|−|LUMO_(H)| representing the strength ofthe electron trapping property(|HOMO_(H)|−|HOMO_(D)|>|LUMO_(D)|−|LUMO_(H)|). As a result, the bluelight-emitting layer 13B has the hole trapping property to suppress themovement of holes, so that it becomes possible to recombine electronsand holes in the blue light-emitting layer 13B efficiently.

In particular, the configuration shown in FIG. 4A can be employed. Inthis case, the blue light-emitting layer 13B satisfies Relationalexpression (9) described below.

|LUMO _(D3) |<|LUMO _(H3) |<|HOMO _(D3) |<|HOMO _(H3)|  (9)

In the present embodiment, red light-emitting layer 13R is configured tosatisfy Relational expression (1) and (2) as with the first embodiment.

Consequently, regarding the red organic electroluminescent element 3R,in the case where the red light-emitting layer 13R has an electrontrapping property, leakage of electrons to the common light-emittinglayer disposed in the anode 11 side of the red light-emitting layer 13Ris prevented and, thereby, the red organic electroluminescent element 3Ris allowed to emit light efficiently. Regarding the blue organicelectroluminescent element 3B, in the case where the blue light-emittinglayer 13B has a hole trapping property, leakage of holes to the commonlight-emitting layer disposed in the cathode 15 side of the bluelight-emitting layer 13B is prevented and, thereby, the blue organicelectroluminescent element 3B is allowed to emit light efficiently.

In the present embodiment, the green light-emitting layer 13G ismentioned as an example of the common light-emitting layer, although notspecifically limited to this. As for the common light-emitting layer,another color light-emitting layer, e.g., a blue light-emitting layer13B or a red light-emitting layer 13R, may also be applied. For example,in the case where the blue light-emitting layer 13B serves as a commonlight-emitting layer, the red light-emitting layer 13R may be configuredto satisfy Relational expressions (1) and (2) described above, and thegreen light-emitting layer 13G may be configured to satisfy Relationalexpressions (7) and (8) described above.

In the above-described example, the red light-emitting layer 13R isdisposed in the cathode 15 side of the common light-emitting layer andthe blue light-emitting layer 13B is disposed in the anode 11 side ofthe common light-emitting layer. However, the red light-emitting layer13R may be disposed in the anode 11 side of the common light-emittinglayer and the blue light-emitting layer 13B may be disposed in thecathode 15 side of the common light-emitting layer. In this case, thered light-emitting layer 13R may satisfy Relational expressions (7) and(8) described above and the blue light-emitting layer 13B may satisfyRelational expressions (1) and (2) described above.

In the present embodiment, the anode 11, the hole transportation layer12, the light-emitting layer, the electron transportation layer 14, andthe cathode 15 are stacked in that order from the substrate 10 side.However, a reverse configuration may be employed.

The display apparatus according to aspects of the present invention maybe a bottom emission type display apparatus in which the light of theorganic electroluminescent element is emitted from the substrate 10side, or be a top emission type display apparatus in which the light ofthe organic electroluminescent element is emitted from the side oppositeto the substrate 10.

EXAMPLES

In the present example, the highest occupied molecular orbital (HOMO)level energy was measured by using photoelectron spectroscopy (AC-2produced by RIKEN KIKI CO., LTD.). The lowest unoccupied molecularorbital (LUMO) level energy was calculated by subtracting the band gap,which was determined from an absorption edge of the absorption spectrummeasured by using ultraviolet and visible spectroscopy (UV/VIS V-560produced by JASCO Corporation), from the HOMO level energy.

Example 1

A display apparatus having the configuration shown in FIGS. 1A and 1Bwas produced. The present example corresponded to the first embodiment.The present example was a bottom emission type display apparatus inwhich the light was taken from the surface in the substrate 10 side.

A low-temperature polysilicon thin film transistor (TFT) was formed on aglass substrate, and an interlayer insulating film made from siliconnitride and a flattening film made from an acrylic resin were formedthereon, so that the substrate 10 shown in FIG. 1A was produced. An ITOfilm having a thickness of 100 nm was formed on the substrate 10 by asputtering method. Subsequently, the ITO film was patterned on a pixelbasis, so as to form an anode 11.

An acrylic resin was formed on the anode 11 through spin coating, andthe acrylic resin was patterned through lithography, so as to form aninsulating layer 20. Ultrasonic cleaning with isopropyl alcohol (IPA)was performed, and cleaning through boiling was performed, followed bydrying. Furthermore, UV/ozone cleaning was performed and, thereafter, anorganic compound layer described below was formed by a vacuumevaporation method under the following configuration.

Initially, Compound 1 having a thickness of 60 nm was evaporated allover the display region 2, so as to form a common hole transportationlayer 12.

A host material represented by Compound 2 and a green light-emittingdopant material represented by Compound 3 were co-evaporated (volumeratio 98:2) on the hole transportation layer 12, so as to form the greenlight-emitting layer 13G having a film thickness of 20 nm all over thedisplay region 2.

Subsequently, a host material represented by Compound 4 and a redlight-emitting dopant material represented by Compound 5 wereco-evaporated (volume ratio 99:1) at a position corresponding to thepixel of the red organic electroluminescent element 3R, so as to formthe red light-emitting layer 13R having a film thickness of 20 nm byusing a mask. Likewise, a host material represented by Compound 6 and ablue light-emitting dopant material represented by Compound 7 wereco-evaporated (volume ratio 95:5) at a position corresponding to thepixel of the blue organic electroluminescent element 3B, so as to formthe blue light-emitting layer 13B having a film thickness of 20 nm byusing a mask.

The HOMO level energy and the LUMO level energy of the Compound 4serving as a host material of the red light-emitting layer 13R were 5.50eV and 2.96 eV, respectively. The HOMO level energy and the LUMO levelenergy of the Compound 5 serving as a red light-emitting dopant materialof the red light-emitting layer 13R were 5.39 eV and 3.22 eV,respectively.

The HOMO level energy and the LUMO level energy of the Compound 6serving as a host material of the blue light-emitting layer 13B were5.68 eV and 2.74 eV, respectively. The HOMO level energy and the LUMOlevel energy of the Compound 7 serving as a blue light-emitting dopantmaterial of the blue light-emitting layer 13B were 5.80 eV and 2.93 eV,respectively.

Compound 8 having a thickness of 10 nm was evaporated all over thedisplay region 2, so as to form a common hole block layer (not shown inthe drawing). Subsequently, Compound 9 having a thickness of 30 nm wasevaporated all over the display region 2, so as to form a commonelectron transportation layer 14.

Then, a thin film of lithium fluoride (LiF) having a thickness of 0.5 nmwas evaporated all over the display region 2, so as to form an electroninjection layer (not shown in the drawing). Thereafter, an aluminummetal having a thickness of 100 nm was evaporated, so as to form a filmof the cathode 15. Finally, the whole display region 2 was sealed with aseal cap 30 including a desiccant in a glove box in a nitrogenatmosphere.

The red light-emitting layer 13R and the blue light-emitting layer 13Bwere light-emitting layers satisfying Relational expressions (1) and (2)and Relational expressions (3) and (4), respectively, and having anelectron trapping property.

The characteristics of the thus obtained display apparatus wereevaluated. When a predetermined current was passed through each of thepixels, the red organic electroluminescent element 3R, the green organicelectroluminescent element 3G, and the blue organic electroluminescentelement 3B exhibited good light emission characteristics of red lightemission, green light emission, and blue light emission, respectively.

Comparative Example 1

In the present comparative example, the same display apparatus as thatin Example 1 except the configurations of the red light-emitting layer13R and the blue light-emitting layer 13B was produced.

The red light-emitting layer 13R having a film thickness of 20 nm wasformed through co-evaporation (volume ratio of 96:4) of the hostmaterial represented by Compound 10 and the red light-emitting dopantmaterial represented by Compound 11. The blue light-emitting layer 13Bhaving a film thickness of 20 nm was formed through co-evaporation(volume ratio of 95:5) of the host material represented by Compound 12and the blue light-emitting dopant material represented by Compound 13.

The HOMO level energy and the LUMO level energy of the Compound 10serving as a host material of the red light-emitting layer 13R were 5.77eV and 2.77 eV, respectively. The HOMO level energy and the LUMO levelenergy of the Compound 11 serving as a red light-emitting dopantmaterial of the red light-emitting layer 13R were 5.16 eV and 2.96 eV,respectively.

The HOMO level energy and the LUMO level energy of the Compound 12serving as a host material of the blue light-emitting layer 13B were5.72 eV and 2.77 eV, respectively. The HOMO level energy and the LUMOlevel energy of the Compound 13 serving as a blue light-emitting dopantmaterial of the blue light-emitting layer 13B were 5.36 eV and 2.49 eV,respectively.

The characteristics of the thus obtained display apparatus wereevaluated. When a predetermined current was passed through each of thepixels, the green organic electroluminescent element 3G exhibited goodlight emission characteristics of green light emission. However,regarding the red organic electroluminescent element 3R and the blueorganic electroluminescent element 3B, insufficient light emissioncharacteristics were exhibited where a monochromatic red light emissioncharacteristic and a monochromatic blue light emission characteristicwere not obtained, but a green light emission component was mixed ineach characteristic.

The reason for this is believed to be that each of the redlight-emitting layer 13R and the blue light-emitting layer 13B is alight-emitting layer configured to have a hole trapping propertysatisfying the relationship, |HOMO_(D)|<|HOMO_(H)| and, in addition,|HOMO_(H)|−|HOMO_(D)|>|LUMO_(D)|−|LUMO_(H)|. That is, it is believedthat leakage of electrons from the red light-emitting layer 13R and theblue light-emitting layer 13B to the common light-emitting layer is notprevented and, thereby, electrons and holes are not recombinedefficiently in the red light-emitting layer 13R and the bluelight-emitting layer 13B.

Example 2

A display apparatus provided with the organic electroluminescent elementhaving the configuration shown in FIG. 3 was produced. The presentexample corresponded to the second embodiment. The present example was atop emission type organic electroluminescent element in which the lightwas taken from the surface opposite to the substrate 10.

The present example is different from Example 1 in the configurations ofthe anode 11 and the cathode 15, the configuration and the formationorder of the blue light-emitting layer 13B, and the configuration of theelectron injection layer. Only portions different from Example 1 will bedescribed below.

The anode 11 was formed from an aluminum alloy and an ITO film.Concretely, in the formation, a film of aluminum alloy having athickness of 200 nm was formed as a reflection electrode, an ITO filmhaving a thickness of 20 nm was formed, and the aluminum alloy and theITO film were patterned on a pixel basis.

In the present example, the blue light-emitting layer 13B was formed inthe anode 11 side while being in contact with the green light-emittinglayer 13G. Concretely, in the step to form the organic compound layer inExample 1, the hole transportation layer 12 was formed. Thereafter, theblue light-emitting layer 13B was formed before the green light-emittinglayer 13G was formed. In this regard, the blue light-emitting layer 13Bhaving a film thickness of 20 nm was formed through co-evaporation(volume ratio of 95:5) of the host material represented by Compound 12described above and the blue light-emitting dopant material representedby Compound 13 described above.

The electron injection layer (not shown in the drawing) having athickness of 60 nm was formed on the electron transportation layer 14through co-evaporation of Compound 9 described above and cesiumcarbonate in such a way that the cesium concentration became 8.3 percentby weight.

Regarding the cathode 15, an IZO film having a thickness of 30 nm wasformed on the electron injection layer by a sputtering method.

The red light-emitting layer 13R was a light-emitting layer satisfyingRelational expressions (1) and (2) and having an electron trappingproperty. The blue light-emitting layer 13B was a light-emitting layersatisfying Relational expressions (7) and (8) and having a hole trappingproperty.

The characteristics of the thus obtained display apparatus wereevaluated. When a predetermined current was passed through each of thepixels, the red organic electroluminescent element 3R, the green organicelectroluminescent element 3G, and the blue organic electroluminescentelement 3B exhibited good light emission characteristics of red lightemission, green light emission, and blue light emission, respectively.

Comparative Example 2

In the present comparative example, the same display apparatus as thatin Example 2 except the configuration of the blue light-emitting layer13B was produced.

The blue light-emitting layer 13B having a film thickness of 20 nm wasformed through co-evaporation (volume ratio of 95:5) of the hostmaterial represented by Compound 6 described above and the bluelight-emitting dopant material represented by Compound 7 describedabove.

The characteristics of the thus obtained display apparatus wereevaluated. When a predetermined current was passed through each of thepixels, the green organic electroluminescent element 3G and the redorganic electroluminescent element 3R exhibited good light emissioncharacteristics of green light emission and red light emission. However,regarding the blue organic electroluminescent element 3B, insufficientlight emission characteristics were exhibited where a monochromatic bluelight emission characteristic was not obtained, but a green lightemission component was mixed in the characteristic.

The reason for this is believed to be that the blue light-emitting layer13B is a light-emitting layer configured to have an electron trappingproperty satisfying the relationship, |LUMO_(D)|>|LUMO_(H)| and, inaddition, |LUMO_(D)|−|LUMO_(H)|>|HOMO_(H)|−|HOMO_(D)|. That is, it isbelieved that leakage of holes to the common light-emitting layerdisposed as a layer on the blue light-emitting layer 13B is notprevented and, thereby, electrons and holes are not recombinedefficiently in the blue light-emitting layer 13B.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-189134 filed Aug. 31, 2011, which is hereby incorporated byreference herein in its entirety.

1. A display apparatus comprising: a first organic electroluminescentelement to emit a first color; and a second organic electroluminescentelement to emit a second color different from the first color, theorganic electroluminescent element being provided with an anode, acathode, and a light-emitting layer disposed between the anode and thecathode, wherein a first light-emitting layer of the first organicelectroluminescent element is disposed in common to the second organicelectroluminescent element, a second light-emitting layer of the secondorganic electroluminescent element is disposed in contact with the firstlight-emitting layer and in the side nearer to the cathode than is thefirst light-emitting layer, and the second light-emitting layer containsa host material and a light-emitting dopant material and is configuredto satisfy Relational expressions (A) and (B) described below,|LUMO _(D2) |>|LUMO _(H2)|  (A)|LUMO _(D2) |−|LUMO _(H2) |>|HOMO _(H2) |−|HOMO _(D2)|  (B) whereLUMO_(H2) and HOMO_(H2) represent the LUMO level energy and the HOMOlevel energy, respectively, of the host material contained in the secondlight-emitting layer and LUMO_(D2) and HOMO_(D2) represent the LUMOlevel energy and the HOMO level energy, respectively, of thelight-emitting dopant material contained in the second light-emittinglayer.
 2. The display apparatus according to claim 1, wherein in thesecond organic electroluminescent element, only the secondlight-emitting layer emits light.
 3. The display apparatus according toclaim 1, wherein the second light-emitting layer is configured tosatisfy Relational expression (G) described below,|HOMO _(D2) |>|HOMO _(H2) |>|LUMO _(D2) |>|LUMO _(H2)|  (G).
 4. Thedisplay apparatus according to claim 1, wherein the first light-emittinglayer emits green light.
 5. The display apparatus according to claim 1,further comprising a third organic electroluminescent element to emit athird color different from the first color and the second color, whereinthe first light-emitting layer is disposed in common to the thirdorganic electroluminescent element as well, a third light-emitting layerof the third organic electroluminescent element is disposed in contactwith the first light-emitting layer and in the side nearer to thecathode than is the first light-emitting layer, and the thirdlight-emitting layer contains a host material and a light-emittingdopant material and is configured to satisfy Relational expressions (C)and (D) described below,|LUMO _(D3) |>|LUMO _(H3)|  (C)|LUMO _(D3) |−|LUMO _(H3) |>|HOMO _(H3) |−|HOMO _(D3)|  (D) whereLUMO_(H3) and HOMO_(H3) represent the LUMO level energy and the HOMOlevel energy, respectively, of the host material contained in the thirdlight-emitting layer and LUMO_(D3) and HOMO_(D3) represent the LUMOlevel energy and the HOMO level energy, respectively, of thelight-emitting dopant material contained in the third light-emittinglayer.
 6. The display apparatus according to claim 5, wherein in thethird organic electroluminescent element, only the third light-emittinglayer emits light.
 7. The display apparatus according to claim 5,wherein the third light-emitting layer is configured to satisfyRelational expression (H) described below,|HOMO _(D3) |>|HOMO _(H3) |>|LUMO _(D3) |>|LUMO _(H3)|  (H).
 8. Thedisplay apparatus according to claim 5, wherein the first light-emittinglayer emits green light, the second light-emitting layer emits redlight, and the third light-emitting layer emits blue light.
 9. Thedisplay apparatus according to claim 1, further comprising a thirdorganic electroluminescent element to emit a third color different fromthe first color and the second color, wherein the first light-emittinglayer is disposed in common to the third organic electroluminescentelement as well, a third light-emitting layer of the third organicelectroluminescent element is disposed in contact with the firstlight-emitting layer and in the side nearer to the anode than is thefirst light-emitting layer, and the third light-emitting layer containsa host material and a light-emitting dopant material and is configuredto satisfy Relational expressions (E) and (F) described below,|HOMO _(D3) |<|HOMO _(H3)|  (E)|HOMO _(H3) |−|HOMO _(D3) |>|LUMO _(D3) |−|LUMO _(H3)|  (F) whereLUMO_(H3) and HOMO_(H3) represent the LUMO level energy and the HOMOlevel energy, respectively, of the host material contained in the thirdlight-emitting layer and LUMO_(D3) and HOMO_(D3) represent the LUMOlevel energy and the HOMO level energy, respectively, of thelight-emitting dopant material contained in the third light-emittinglayer.
 10. The display apparatus according to claim 9, wherein in thethird organic electroluminescent element, only the third light-emittinglayer emits light.
 11. The display apparatus according to claim 9,wherein the third light-emitting layer is configured to satisfyRelational expression (I) described below,|LUMO _(D3) |<|LUMO _(H3) |<|HOMO _(D3) |<|HOMO _(H3)|  (I).
 12. Thedisplay apparatus according to claim 9, wherein the first light-emittinglayer emits green light, the second light-emitting layer emits redlight, and the third light-emitting layer emits blue light.
 13. An imagepickup apparatus comprising the display apparatus according to claim 1and an image pickup element.